BuddyPress.org

Try this:

function change_bp_new_message_email_content($email_content, $sender_name, $subject, $content, $message_link, $settings_link) {

	$email_content = sprintf( __(
	'You have one new message:
	Subject: %1$s
	"%2$s"
	To view and read your messages please log in and visit: %3$s
	---------------------
	', 'buddypress' ), $subject, $content, $message_link );
	
	return $email_content;

}
add_filter('messages_notification_new_message_message', 'change_bp_new_message_email_content', 1,  6);